Greetings, members of the WoW community!
The confusion about the new cinematic is rather obvious at this point, and I do have some of my own critiques about it. However, I do believe that if we look at the recent lore of Elune and in Shadowlands, we can piece it together into a reasonable story experience. Note: this information comes solely from presented lore with minimal speculation Here we go!
First off: Elune. As a goddess, she has NEVER been directly involved in a major way. The closest we’ve seen is her intercepting Ysera’s soul. Not during the War of the Ancients, not during the third War, not during Cataclysm, Legion or anything like that. That is proven to exhaustion by now.
We know, for a fact, that the Winter Queen HATES Elune. She does not like her. It is heavily implied that Elune walked out on the Winter Queen for reasons yet unknown. Elune was not around, and the Winter Queen was not going to be talking to her to fill in what’s going on.
The Winter Queen accused Elune of ignoring her “cries for help.” This means more than one time, the Winter Queen tried to get Elune to help. Yet Elune states that she “heard her cries, felt her pain.” I believe that, due to Elune not being able to get involved due to not being ABLE to rather than not choosing to, she was trying to find ways to help the Winter Queen. (This is the "minimal speculation part.)
Elune stated that she, “in the wake of tragedy, sent forth the cascade of souls to sustain you.” To me, this message is clear: Elune did NOT cause the burning of Teldrassil. She was deeply grieved by it, seeing she referred to it as a tragedy and actually cried. But as shown with her intercepting Ysera’s soul, she believed she could take the kaldorei souls to Ardenweald so they could help the Winter Queen’s realm survive. She could use this chance to help her sister, grim as it is, but she would also be allowing her children to rest.
She could not intervene to stop the burning, but she would let their deaths have meaning.
But due to the Winter Queen not knowing about the Arbiter being broken until we quest in Shadowlands (long after the cries for help were sent out), and Elune was off somewhere else and on the Winter Queen’s bad side (and thus not on speaking terms), she accidentally sent the souls to the Maw when she wanted them to go to Ardenweald. This was clearly not her intent, as that is when she cried.
That is what I interpreted the cinematic as. TL;DR, Elune couldn’t stop the burning, so she tried to help her sister in the grim aftermath of the burning. Yet her separation caused her to not realize where the souls were going.
These are my thoughts to explain Elune’s actions. She’s shown time and time again very limited intervention, but this may be because she cannot. I can bet that if she could have saved the Kaldorei, she would have. But she couldn’t, and so she tried to guide them to a greater purpose. She is depicted as well-meaning for her limited capability, and yet relatable in that she can make mistakes. She truly desired to help the sister that shunned her, yet she was as ignorant as the rest at the time (barring the Primus and Denathrius) and accidentally condemned her followers to the Maw.
I hope this all makes a bit more sense now.